For solid-state lighting applications, the fastest route to high efficiency white LEDs is to combine either blue, violet, or near UV LEDs with appropriate phosphors. The prototypical conventional phosphor is Ce3+-doped Y3Al5O12 (YAG:Ce), a yellow emitter which in its commercial form from Nichia has a quantum efficiency of 70%. Recently, progress has been made in creating highly efficient green-yellow and red phosphors using nitridosilicates (R. Mueller-Mach et al., Phys. Stat. Sol. A, 202 (9), 1727 (2005)). Despite the very good quantum efficiencies of conventional phosphors, they suffer from enhanced optical backscattering due to their large size and it is difficult to tune their emission response in order to get spectra with specific correlated color temperatures (CCT) having high color rendering index (CRI) values.
A way to overcome the backscattering loss issue is to form colloidal quantum dot phosphors. As is well known the crystallinity of colloidal quantum dots can be made to be very high which results in solution quantum yields being 80-90%, and sometimes nearly 100% (J. McBride, et al., Nano Lett. 6 (7), 1496 (2006)). In addition to the reduced scattering losses, colloidal quantum dot phosphors also enjoy the advantages of ease of color tuning, improved CRI, a lower cost deposition process, and a broader wavelength spectrum for optical pumping. Despite these advantages, colloidal quantum dot phosphors have not been introduced into the marketplace due to two major shortcomings; namely, poor temperature stability (thermal quenching of quantum efficiency) (N. Pradhan et al., J. Am. Chem. Soc. 127 (50), 17586 (2005)) and low (10-20%) quantum yields for phosphor films with high quantum dot packing densities. One way to get around the non-ideal temperature stability of colloidal quantum dots is to dope the nanocrystals with impurity atoms, as was done by Peng and co-workers (N. Pradhan et al., J. Amer. Chem. Soc. 129, 3339 (2007)), where it was found that Mn-doped ZnSe nanocrystals maintained a reasonable thermal stability up to ˜250° C. The disadvantages of this approach are that the peak emission wavelengths of the nanocrystals are limited by the particular choice of dopant materials, the spectral widths of the photoluminescence are typically larger for impurity emission, and the quantum efficiency of these types of nanocrystals is below that of undoped nanocrystals.
Turning back to the undoped nanocrystals, an important channel for non-radiative energy decay is the transfer of the carriers (electron or hole) or exciton energy to the surface defects (D. Berasis et al., “Luminescent Materials and Applications”, 2008 John Wiley & Sons Ltd., pg. 19). This pathway is enhanced at higher temperatures since the electron and hole wavefunctions will overlap with the surface region to a greater extent at higher temperatures. One way to minimize the overlap of the electrons and holes with the surface impurities is to grow nanocrystals with very thick shells (Y. Chen et al., J. Am. Chem. Soc., 130 (15), 5026 (2008)). The problems with this approach are that the shell growth times can be prohibitively long and the quantum efficiency tends to fall for very thick shells due to their greater propensity for defect formation. One way for reducing the thickness of the shell, while increasing the quality of the shell growth is to use outer shells of the widest bandgap, such as, ZnS for CdSe, while employing a graded shell interface region to enable a smooth transition from the core to the shell regions, for example, varying from the CdSe-like core to the ZnS-like outer shell region (K. Char et al., U.S. Patent Application Publication 2010/0140586; S. Weiss et al., U.S. Patent Application Publication 2008/0064121; and J. Treadway et al., WO 2003/092043). Though a reasonable way for preventing the electrons and holes from feeling the effects of the nanocrystal surface, in all three of these cases the core regions are at least 2 nm in diameter; which are typical of quantum dots, and thus would show the typical thermal responses associated with core diameters of these sizes.
To date, traditional (not employing impurity dopants) nanocrystals suffer from poor thermal stability which limits the usefulness of these materials in high temperature applications, such as, high power LEDs and nanocrystal-based lasers. Some nanocrystals have been engineered for minimizing the impact of the shell surface states on the radiative recombination of the electrons and holes. However, the engineered nanocrystals had other problems, such as, reduced quantum efficiencies at room temperature. As such, there is a need for a new class of colloidal nanocrystals, which have very good quantum efficiencies at room temperature, and maintain these efficiencies at elevated temperatures.